In the power generation and distribution industry, utility companies generate and distribute electricity to customers. To facilitate the process of distributing electricity, various types of power switching devices are used. In a distribution circuit, electricity flows through the power switching devices from a power generation source (a substation or the like) to the consumer. When a fault is detected in the distribution circuit, the power switching device is opened and the electrical connection is broken.
Various controllers and protective relays are used by the utility company to detect faults that occur in the distribution circuit. This most controllers use a microprocessor programmed to respond to the fault based on the type of fault and the type of power switching device connected to the controller. The controller may respond to a particular fault by causing the power switching device to open. Alternatively, upon the detection of a fault, the controller may cause the power switching device to open and close multiple times.
In order to make the most efficient switching in the distribution circuit and isolate the fault, the controllers need to monitor both the voltage present at the power switching device and electrical current flowing through the power switching device. If the amount of current exceeds a preprogrammed threshold for a certain period of time, the controller instructs the power switching device to perform the preprogrammed response. Should the fault continue to persist, the power switching device opens and remains open.
Monitoring the voltage levels at the power switching device is essential for determining, for example, the direction of power flow, if the power switching device is being back-fed, or if the three phases of power are synchronized. Additionally, the utility personnel can use this information to monitor the output and efficiency of the distribution transformers providing power through the power switching devices. Presently, utility company personnel monitor voltage levels present at the power switching device by using dedicated potential transformers that are connected to the power switching devices. The controllers sample the output of the potential transformers and report this information to the craftsperson or other utility personnel. Voltage levels may be monitored at both the input connector and the output connector of the power switching device.
Using a dedicated potential transformer as a voltage measuring device is cumbersome and expensive because each voltage phase must be monitored separately. If voltage is measured at both connectors, two dedicated potential transformers are required per phase and there may not be enough room on the utility pole for each of the potential transformers. One solution is to use a voltage divider circuit connected to a conductor of a power device. The voltage divider circuit can be designed to include resistors or capacitors. Typically, the voltage drop over a divider load impedance (low voltage leg) is measured with respect to a voltage drop over a reference impedance (high voltage leg). From this ratio a value of the voltage potential is determined.
The use of a capacitive voltage divider in a power switching device to measure voltage is described in U.S. Pat. No. 4,074,193 (“the '193 patent”). The '193 patent discloses the use of a separate cylindrical conductor as an electrode forming in part the high voltage leg of a capacitive voltage divider. The corresponding voltage from the low voltage leg of the capacitive voltage divider is amplified and sent to a voltage potential measuring device, meter or controller.
The present invention eliminates the need for a separate dedicated conductor for the high voltage leg of the capacitive voltage divider. The present invention instead uses an existing shield of a current measuring device such as, for example, a transformer or Rogowski coil in the power switching device. The capacitive relationship between the shield and the high voltage conductor form the high voltage leg of the capacitive voltage divider. By using the existing shield, the cost of the separate conductor is eliminated. The present invention also allows the capacitive voltage divider to be tuned to thereby provide greater accuracy in measuring the voltage potential at the power switching device.